Rev. Proc. 2010-23: Qualified mortgage bonds; mortgage credit certificates; national median gross income. This revenue procedure provides guidance with respect to the United States and area median gross income figures that are to be used by issuers of qualified mortgage bonds, as defined in § 143(a) of the Internal Revenue Code, and issuers of mortgage credit certificates, as defined in § 25(c), in computing the housing cost/income ratio described in § 143(f)(5).